1. Field of the Invention
The invention relates to the use of special materials for female dies and comparable construction components for extruding light and nonferrous metals.
2. Description of the Related Prior Art
The quality of female die materials for such applications is measured against the following important requirements:
High accuracy to size, i.e., high yield point and high creeping strength at the operating temperature of the die; PA1 low susceptibility to thermomechanical fatigue or formation of cracks (an advantageous precondition for this is high thermal conductivity of the material); PA1 high surface quality or low surface roughness of the extruded material; PA1 applicability for high pressing rates; PA1 adequate cold ductility; and PA1 high resistance to erosion/corrosion.
Primarily, hot-working steels, nickel-based superalloys and stellites have been used heretofore as die materials for such applications. Said materials have a comparatively low thermal conductivity and, therefore, are susceptible to thermomechanically induced fracture cracks. The surface quality of the extruded material obtained with the application of such female dies is much in need of improvement, for example as compared to the surface quality produced by means of ceramic female dies.
But the known drawbacks of the ceramic material, especially the low ductility and particularly the cold ductility highly limit the field of application for ceramic female dies as well.
DE-AS 17 58 508 describes the application of a composite material consisting of 20 to 85% by volume molybdenum and/or tungsten as the metallic component, the balance being zirconium oxide as the oxide-ceramic component, as material for the manufacture of female dies for extruding nonferrous and light metals.
The extruded material produced with such female dies is characterized by very good surface quality. A drawback is the not-always sufficient resistance to heat and creeping strength, and thus the early failure of the female dies.
Molybdenum alloys having the composition Mo, 1.2% Hf, 0.1% C or Mo, 0.5% Ti, 0.08% Zr, 0.02 to 0.04% C are used as female die materials for extruding copper alloys, whereby the applicability had to be limited to copper alloys with a copper content of &lt;70% by weight.
The extrusion of light and nonferrous metals alloyed in different ways failed on account of the low resistance to erosion of said material; in particular, undesirable reactions of the extruded material with the female die material occurred.
It is known, for example from DE-AS 17 58 923 and DE-AS 17 58 924, to enhance the resistance to abrasion of "workpieces" consisting of metal alloys through superficial nitriding, whereby the metal alloys contain metals of three groups: niobium, tantalum and vanadium, in the one group, molybdenum and tungsten in the second group, and titanium in the third. The application of such "workpieces" as tools, especially as cutting tools, has been concretely stated and tested in the above-mentioned published references. The great number of materials explicitly mentioned therein includes a few molybdenum-containing materials as well, with a molybdenum content of 60% by weight at the most, preferably with molybdenum component of less than 45% by weight.
According to the patent specification, such materials are provided by such superficial nitriding with "certain mechanical properties, in particular resistance to wear", such properties having an effect in cutting tests of cutting tools made of such materials. Female extrusion dies with quality requirements highly different versus cutting tools have not been mentioned in said references.
Based on the general technical importance of surface nitriding for changing the surface properties of metallic materials, the nitriding of molybdenum materials has been more recently described with greater systematics in a paper by H. P. Martinz in the "Proceedings of the 13th International PLANSEE Seminar 1993", Vol. I, pp 632 ff. The paper demonstrates that the nitriding of molybdenum materials, as opposed to, for example the nitriding of iron materials, comprises a great number of different nitriding reactions, but also adverse reactions depending on the process conditions, which are specified in detail. The property changes in molybdenum alloys caused by nitriding, furthermore, have not been substantially elucidated by said work paper. They are substantially limited in the latter to the finding that nitride layers do not increase the resistance to oxidation of molybdenum at temperatures above 640.degree. C.